1. Field of the Invention
The present invention relates to an apparatus and method for image-compression encoding and decoding using an adaptive transform, and more particularly to an apparatus and method for image-compression encoding and decoding using an adaptive transform in which, where different transform coefficients are outputted in accordance with a change of the transform direction order for an input image signal block, encoding and decoding procedures are conducted, based on the transform direction order selected in accordance with the characteristics of the input image signal block.
2. Description of the Prior Art
Among compression encoding schemes for digital images, the scheme most commonly used is a scheme in which every block of an image signal is transformed into a block having transform coefficients in accordance with an orthogonal transform method, and the transform coefficients are encoded. This scheme is widely used in a scheme proposed by Joint Technical Committee Joint Photographic Coding Experts Group (JPEG) of International Standardization Organization/International Electrotechnical Commission (ISO/IEC) JTC1/SC29/WG1, which is a compression encoding standardization scheme for still images, a scheme proposed by Moving Picture Experts Group (MPEG) of ISO/IEC, which is a compression encoding standardization scheme for moving images, and other international standardization schemes for digital compression encoding such as H.261 of International Telecommunication Unionxe2x80x94Telecommunication Standardization Sector (ITU-T) and H.263 of ITU-T.
In accordance with conventional transform-based image signal compression techniques, every block of an input image signal is transformed into a block having transform coefficients in accordance with an orthogonal transform method. The transform coefficients are quantized, and then processed in accordance with a variable-length encoding method. The resultant signal is then transmitted to a decoder unit. In the decoder unit, the transmitted signal is decoded in accordance with processes inverse to those used in the encoding method, thereby recovering the original image signal. Referring to FIG. 1, a conventional image compression encoding and decoding system is illustrated. As shown in FIG. 1, the system mainly includes an encoding unit for encoding blocks of an input image signal using a compression technique, and a decoding unit for receiving an encoded signal outputted from the encoding unit, and conducting a decoding procedure, inverse to the encoding procedure, for the encoded signal, thereby recovering the original image signal.
The encoding unit includes a subtractor 1 for conducting a subtraction between an input image signal block and an image signal block, previously recovered, thereby outputting an residual signal block, and an overhead information encoder 9 for outputting an overhead information signal (for example, shape information) and a variable-length-coded shape information signal, which are to be used in the encoding procedure. The encoding unit also includes an orthogonal transform unit 2 for receiving the residual signal block from the subtractor 1 while selectively receiving the shape information from the overhead information encoder 9 in accordance with a switching operation conducted by a switch 10b, and performing a transform for the residual signal block based on the shape information in accordance with a specific transform method, thereby outputting a transform coefficient block, and a quantizer 3 for receiving the transform coefficient block from the orthogonal transform unit 2, and quantizing the received transform coefficient block into a quantized transform coefficient block. The encoding unit further includes a variable-length encoder 8 for receiving the quantized transform coefficient block from the quantizer 3, and encoding the received quantized transform coefficient block into a variable-length-coded transform coefficient signal, and a multiplexer 11 for receiving the variable-length-coded signal from the variable-length encoder 8 while selectively receiving the shape information from the overhead information encoder 9 in accordance with a switching operation conducted by a switch 10a, multiplexing the received signals together, and transmitting the resultant signal to a transmission medium 12.
The encoding unit also includes a first inverse quantizer 4 for receiving the quantized transform coefficient block from the quantizer 3, and conducting a inverse quantization for the received block, a inverse orthogonal transform unit 5 for receiving the inverse-quantized transform coefficient block from the first inverse quantizer 4 while selectively receiving the shape information from the overhead information encoder 9 in accordance with a switching operation of the switch 10b, and transforming the received block into a inverse-orthogonal-transformed signal block based on the shape information, an adder 6a for adding the previously recovered image signal block to the inverse-orthogonal-transformed signal block outputted from the inverse orthogonal transform unit 5, thereby recovering the input image signal block, and a first memory 7 for receiving the recovered image signal block outputted from the adder 6a, and outputting it to the subtractor 1 as the previously recovered image signal block.
On the other hand, the decoding unit includes a demultiplexer (DEMUX) 13 for receiving the variable-length-encoded signal from the multiplexer 11 via the transmission medium 12, thereby outputting a variable-length-encoded shape information signal and a variable-length-encoded input transform coefficient signal, a decoder 15 for decoding the variable-length-encoded coefficient signal into a quantized transform coefficient block, and a second inverse quantizer 16 for receiving the quantized transform coefficient block from the decoder 154, and inverse quantizing the received block into a inverse-quantized transform coefficient block. The decoding unit also includes an overhead information decoder 14 for receiving the variable-length-encoded shape information from the demultiplexer 13, and decoding the received shape information, thereby outputting recovered shape information, and a inverse orthogonal transform unit 17 for receiving the inverse-quantized transform coefficient block from the second inverse quantizer 16 while selectively receiving the shape information from the overhead information decoder 14 in accordance with a switching operation conducted by a switch 19, and performing a inverse orthogonal transform for the received transform coefficient block based on the received shape information, thereby outputting a inverse-orthogonal-transformed signal block. The decoding unit further includes an adder 6b for adding an image signal block, previously recovered, to the inverse-orthogonal-transformed signal block outputted from the inverse orthogonal transform unit 17, thereby outputting a recovered image signal block, and a second memory 18 for receiving the recovered image signal block outputted from the adder 6b, and outputting it to the adder 6b as the previously recovered image signal block.
The shape information is information for sorting an image into an object field and a background field. Such shape information makes it possible to allow a signal processing, associated with signal encoding and decoding, to be conducted based on the object field of an image, instead of the entire field of the image. Generally, shape information has the form of a binary mask consisting of pixels including object pixels and non-object pixels, that is, background pixels, having a value different from that of the object pixels.
Now, operations of the convention image compression-encoding and decoding system having the above mentioned configuration will be described.
When the subtractor 1 outputs an residual signal block after conducting a subtraction between an input image signal block and a recovered image signal block, the residual signal block is applied to the orthogonal transform unit 2 which also selectively receives shape information from the overhead information encoder 9. Based on the shape information, the orthogonal transform unit 2 performs a transform for the residual signal block in accordance with a specific transform method, thereby outputting a transform coefficient block. The subtractor 1 may output the same signal as the input image signal, instead of the residual between the predicted signal outputted from the first memory 7 and the current input signal. In other words, the subtractor 1 may output the current input signal, as it is, without any prediction based on the output signal from the first memory 7. This may be achieved by initializing the value stored in the first memory 7 in such a fashion that it corresponds to xe2x80x9c0xe2x80x9d.
The quantizer 3 receives the transform coefficient block from the orthogonal transform unit 2, and quantizes the received transform coefficient block into a quantized transform coefficient block which is, in turn, sent to the variable-length encoder 8. In the variable-length encoder 8, the quantized transform coefficient block is transformed into a variable-length-coded transform coefficient signal which is, in turn, applied to the multiplexer 11.
The multiplexer 11 receives the variable-length-coded signal while selectively receiving the shape information from the overhead information encoder 9, and multiplexes the received signals together. The resultant signal from the multiplexer 11 is transmitted to the transmission medium 12.
The demultiplexer 13 receives the variable-length-encoded signal from the transmission medium 12, thereby outputting a variable-length-encoded shape information signal and a variable-length-encoded input transform coefficient signal.
The decoder 15 receives the variable-length-encoded coefficient signal from the demultiplexer 13, and decodes the received signal into a quantized transform coefficient block. This quantized transform coefficient block is applied to the second inverse quantizer 16 which, in turn, inverse quantizes the quantized transform coefficient block into a inverse-quantized transform coefficient block.
The inverse orthogonal transform unit 17 receives the inverse-quantized transform coefficient block from the second inverse quantizer 16 while selectively receiving the shape information from the overhead information decoder 14, thereby outputting a inverse-orthogonal-transformed signal block. The adder 6b receives the inverse-orthogonal-transformed signal block along with an image signal block, previously recovered, from the second memory 18, and performs an addition of those received signals, thereby outputting a recovered image signal block.
On the other hand, the quantized transform coefficient block outputted from the quantizer 3 is also applied to the first inverse quantizer 4 which, in turn, outputs a inverse-quantized transform coefficient block. This inverse-quantized transform coefficient block is applied to the inverse orthogonal transform unit 5.
The inverse orthogonal transform unit 5 also selectively receives the shape information from the overhead information encoder 9. Based on the received signals, the inverse orthogonal transform unit 5 outputs a inverse-orthogonal-transformed signal block. The adder 6a adds the previously recovered image signal block outputted from the first memory 7 to the inverse-orthogonal-transformed signal block, thereby outputting a recovered image signal block. This recovered image signal block is stored in the first memory 7 which, in turn, applies the stored image signal block to the subtractor 1 when a next image signal block is inputted.
The overhead information decoder 14 outputs recovered shape information. Meanwhile, the second memory 18 is stored with the recovered image signal block therein for a next prediction.
Now, the transform method used in the orthogonal transform unit 2 will be described in conjunction with FIGS. 2a to 2e. In FIG. 2a, the shaded pixels of an image signal block are object pixels to be encoded. In accordance with the transform method, only these object pixels are transformed so that they are encoded. When an image signal block of FIG. 2a is inputted, a pixel rearrangement is carried out for the input image signal block in accordance with the transform method. That is, the object pixels of the input image signal block are vertically shifted to the upper border of the block, thereby filling that border, as shown in FIG. 2b. In this state, a discrete cosine transform (DCT) is performed in a vertical direction, as shown in FIG. 2c. 
Thereafter, a pixel rearrangement is carried out again for the image signal block of FIG. 2c by shifting again the object pixels to the left border of the block. Thereafter, DCT is performed in a horizontal direction, thereby obtaining a finally transformed signal as shown in FIG. 2e. 
FIGS. 3a to 3c are views illustrating a transform conducted in the orthogonal transform unit 2 for an input image signal block. FIG. 3a shows the input image signal block applied to the orthogonal transform unit 2. In FIG. 3a, the portions bearing no numeric value correspond to pixels having no object to be encoded, that is, background pixels, whereas the portions bearing numeric values correspond to object pixels to be transformed, that is, to be encoded.
FIG. 3b shows transform coefficients obtained after orthogonal-transforming the input image signal block in a vertical direction, and then in a horizontal direction. On the other hand, FIG. 3c shows transform coefficients obtained after orthogonal-transforming the input image signal block in a horizontal direction, and then in a vertical direction.
After a comparison of FIGS. 3b and 3c, it can be found that different transform results, that is, different transform coefficients, are obtained in accordance with different orthogonal transform directions, respectively.
Referring to FIGS. 3a to 3c, it can be found that when an orthogonal transform is conducted for an input image signal block exhibiting an higher similarity in a horizontal direction, as shown in FIG. 3a, the result of FIG. 3c obtained after an orthogonal transform conducted for the input image signal block in a horizontal direction and then in a vertical direction exhibits a high energy concentration effect, as compared to the result of FIG. 3b obtained after an orthogonal transform conducted for the input image signal block in a vertical direction and then in a horizontal direction. In other words, the orthogonal transform method of FIG. 3c exhibits an increased compression performance because of a reduction in the number of transform coefficients to be encoded.
Where the input image signal block is orthogonal-transformed in a vertical direction and then in a horizontal direction, as shown in FIG. 3b, it is necessary to encode 12 different transform coefficients. However, where the input image signal block is orthogonal-transformed in a horizontal direction and then in a vertical direction, as shown in FIG. 3c, it is necessary to encode only 5 different transform coefficients. Accordingly, it is possible to obtain an increased encoding efficiency in the latter case.
In conventional orthogonal transform methods, an input image signal is simply orthogonal-transformed in a horizontal direction and then in a vertical direction, or in a vertical direction and then in a horizontal direction, without taking into consideration the characteristics thereof, that is, the similarity. As a result, these conventional orthogonal transform methods have disadvantages in that it is impossible to obtain optimum transform results.
The present invention has been made in view of the above mentioned problems, and, therefore, an object of the invention is to provide an image compression-encoding and decoding apparatus and method using an adaptive transform which are capable of obtaining a high compression-encoding efficiency.
In accordance with one aspect, the present invention provides an image compression-encoding apparatus for performing an orthogonal transform for blocks of an input image signal in accordance with an adaptive transform method, thereby compressing the input image signal blocks, comprising: transform mode control means for selectively receiving transform coefficients, obtained after an orthogonal transform for blocks of an input image signal, and an image signal block to be currently encoded, determining correlations of the input values in horizontal and vertical directions, and generating a transform direction control signal based on the determined correlations, the transform direction control signal being adapted to control an orthogonal transform for the current image signal block in such a fashion that the current image signal block is orthogonal-transformed in a direction involving a higher correlation, and then in a inverse direction; orthogonal transform means for receiving the transform direction control signal from the transform mode control means, and performing an orthogonal transform based on the received transform direction control signal.
In accordance with another aspect, the present invention provides an image compression-encoding apparatus for performing an orthogonal transform for blocks of an input image signal in accordance with an adaptive transform method, thereby compressing the input image signal blocks, comprising: transform mode control means for receiving a recovered image signal block while selectively receiving an image signal block to be currently encoded and selectively receiving shape information associated with the image signal blocks, determining correlations of the input values in horizontal and vertical directions, and generating a transform direction control signal based on the determined correlations, the transform direction control signal being adapted to control an orthogonal transform for the current image signal block in such a fashion that the current image signal block is orthogonal-transformed in a direction involving a higher correlation, and then in a inverse direction; and orthogonal transform means for receiving the transform direction control signal from the transform mode control means, and performing an orthogonal transform based on the received transform direction control signal. In accordance with another aspect, the present invention provides An image compression-encoding apparatus using an adaptive transform method comprising: a subtractor for conducting a subtraction between an input image signal block, to be encoded, and an image signal block, previously recovered, thereby outputting an residual signal block; an overhead information encoder for outputting shape information to be used in an encoding procedure; transform mode control means for receiving a quantized transform coefficient while selectively receiving the residual signal block from the subtractor, thereby outputting an associated transform direction control signal; orthogonal transform means for receiving the residual signal block from the subtractor while selectively receiving the shape information from the overhead information encoder, and performing an orthogonal transform for the residual signal block while being controlled in transform direction by the transform mode control means, thereby outputting a transform coefficient; a quantizer for receiving the transform coefficient from the orthogonal transform means, quantizing the received transform coefficient, and outputting the quantized transform coefficient to the transform mode control means and other elements of the apparatus; a variable-length encoder for receiving the quantized transform coefficient from the quantizer, and transforming the received quantized transform coefficient into a variable-length-coded signal; and a multiplexer for receiving the variable-length-coded signal from the variable-length encoder while selectively receiving the shape information from the overhead information encoder, multiplexing the received signals together, and transmitting the resultant signal to a transmission medium. In accordance with another aspect, the present invention provides an image compression-decoding apparatus for decoding a compression-encoded image signal transmitted thereto comprising: inverse transform mode control means for selectively receiving a signal block currently processed by a variable length decoding procedure after being transmitted while receiving a signal block previously variable-length-decoded, determining correlations of the received blocks in horizontal and vertical directions, and generating a inverse transform direction control signal based on the determined correlations, the inverse transform direction control signal being adapted to control a inverse orthogonal transform for the current signal block in such a fashion that the current signal block is inverse-orthogonal-transformed in a direction involving a lower correlation, and then in a inverse direction; and inverse orthogonal transform means for receiving the inverse transform direction control signal from the inverse transform mode control means, and performing a inverse orthogonal transform for the currently-variable-length-decoded signal block based on the received inverse transform direction control signal.
In accordance with another aspect, the present invention provides an image compression-decoding apparatus using an adaptive transform method comprising: a demultiplexer for demultiplexing an input encoded signal received from an encoding unit, thereby outputting an encoded image signal block and encoded shape information; a decoder for receiving the encoded image signal block from the decoder, and variable-length-decoding the received image signal block; a inverse quantizer for receiving the variable-length-decoded signal block from the decoder, and inverse-quantizing the received signal block; an overhead information decoder for selectively receiving the encoded shape information from the demultiplexer, and recovering original shape information from the received shape information; inverse transform mode control means for selectively receiving a current variable-length-decoded signal block from the decoder while receiving a previous variable-length-decoded signal block, determining correlations of the received signal blocks in horizontal and vertical directions, and generating a inverse transform direction control signal based on the determined correlations, the inverse transform direction control signal being adapted to control a inverse orthogonal transform for the current signal block in such a fashion that the current signal block is inverse-orthogonal-transformed in a direction involving a lower correlation, and then in a inverse direction; inverse orthogonal transform means for receiving the inverse-quantized signal from the inverse quantizer while selectively receiving the recovered shape information from the overhead information decoder, and performing a inverse orthogonal transform for the inverse-quantized signal block under a control of the inverse transform mode control means; an adder for receiving the inverse-orthogonal-transformed signal block from the inverse orthogonal transform means while receiving an image signal block previously recovered, and adding the received signal blocks, thereby outputting a recovered image signal block; and a memory for storing the recovered image signal block outputted from the adder, and outputting the stored imaged signal block to the adder for an prediction.
In accordance with another aspect, the present invention provides an image compression-encoding and decoding system using an adaptive transform method comprising:
an encoding unit comprising
a subtractor for conducting a subtraction between an input image signal block, to be encoded, and a image signal block, previously recovered, thereby outputting an residual signal block,
an overhead information encoder for outputting shape information to be used in an encoding procedure, along with a variable-length-encoded shape information signal;
mode control means for receiving the residual signal block, to be encoded, from the subtractor while receiving the shape information signal from the overhead information encoder, thereby outputting an associated transform direction control signal,
orthogonal transform means for receiving the residual signal block from the subtractor while selectively receiving the variable-length-encoded shape information from the overhead information encoder, and performing an orthogonal transform for the residual signal block while being controlled in transform direction by the mode control means, thereby outputting a transform coefficient,
a quantizer for receiving the transform coefficient from the orthogonal transform means, and quantizing the received transform coefficient, thereby outputting a quantized transform coefficient,
a variable-length encoder for receiving the quantized transform coefficient from the quantizer, and transforming the received transform coefficient into a variable-length-coded signal,
a control signal encoder for receiving the transform direction control signal from the mode control means, and encoding the received transform direction control signal, and
a multiplexer for receiving the variable-length-coded signal from the variable-length encoder and the transform direction control signal from the control signal encoder while selectively receiving the shape information from the overhead information encoder, multiplexing the received signals together, and transmitting the resultant signal to a transmission medium; and
a decoding unit comprising
a demultiplexer for demultiplexing the signal received from the encoding unit via the transmission medium, thereby outputting the variable-length-coded shape information, the variable-length-coded quantized transform coefficient block, and the encoded transform direction control signal,
an overhead information decoder for selectively receiving the encoded shape information from the demultiplexer, and recovering original shape information from the received shape information,
a decoder for receiving the variable-length-encoded quantized transform coefficient from the decoder, and variable-length-decoding the received transform coefficient, thereby outputting a variable-length-decoded quantized transform coefficient block,
a first inverse quantizer for receiving the variable-length-decoded quantized transform coefficient block, and inverse-quantizing the received transform coefficient block,
inverse orthogonal transform means for selectively receiving the inverse-quantized transform coefficient block from the first inverse quantizer while receiving the shape information from the overhead information decoder and the transform direction control signal from the control signal decoder, and inverse-orthogonal-transforming the inverse-quantized transform coefficient block in a direction inverse to that of the orthogonal transform means,
a first adder for receiving the inverse-orthogonal-transformed signal block from the inverse orthogonal transform means while receiving an image signal block previously recovered, and adding the received signal blocks, thereby outputting a recovered image signal block, and
a first memory for storing the recovered image signal block outputted from the first adder, and outputting the stored imaged signal block to the adder for an prediction.
In accordance with another aspect, the present invention provides an image compression-encoding and decoding system using an adaptive transform method comprising:
an encoding unit comprising
a subtractor for conducting a subtraction between an input image signal block, to be encoded, and a image signal block, previously recovered, thereby outputting an residual signal block,
an overhead information encoder for outputting shape information to be used in an encoding procedure, along with a variable-length-encoded shape information signal;
mode control means for receiving the residual signal block, to be encoded, from the subtractor while receiving the shape information signal from the overhead information encoder, thereby outputting an associated transform direction control signal,
orthogonal transform means for receiving the residual signal block from the subtractor while selectively receiving the shape information from the overhead information encoder, and performing an orthogonal transform for the residual signal block while being controlled in transform direction by the mode control means, thereby outputting a transform coefficient,
a quantizer for receiving the transform coefficient from the orthogonal transform means, and quantizing the received transform coefficient, thereby outputting a quantized transform coefficient,
a variable-length encoder for receiving the quantized transform coefficient from the quantizer, and transforming the received transform coefficient into a variable-length-coded signal,
a control signal encoder for receiving the transform direction control signal from the mode control means, and encoding the received transform direction control signal,
mode controller control signal generating means adapted to generate a mode controller control signal for controlling respective ON/OFF operations of the mode control means and the control signal encoder in accordance with whether or not the transform direction control signal is used in the orthogonal transform or inverse orthogonal transform,
a mode controller control signal encoder for receiving the mode controller control signal from the mode controller control signal generating means, and encoding the received signal, and
a multiplexer for receiving the variable-length-coded signal from the variable-length encoder, the encoded transform direction control signal from the control signal encoder, and the encoded mode controller control signal from the mode controller control signal encoder while selectively receiving the shape information from the overhead information encoder, multiplexing the received signals together, and transmitting the resultant signal to a transmission medium; and
a decoding unit comprising
a demultiplexer for demultiplexing the signal received from the encoding unit via the transmission medium, thereby outputting the variable-length-coded shape information, the variable-length-coded quantized transform coefficient, the variable-length-encoded transform direction control signal, and the encoded mode controller control signal,
an overhead information decoder for selectively receiving the encoded shape information from the demultiplexer, and recovering original shape information from the received shape information,
a decoder for receiving the variable-length-encoded quantized transform coefficient from the decoder, and variable-length-decoding the received transform coefficient, thereby outputting a variable-length-decoded quantized transform coefficient block,
control signal decoder for receiving the variable-length-encoded transform direction control signal from the demultiplexer, and decoding the received transform direction control signal,
mode controller control signal decoder for receiving the encoded mode controller control signal from the demultiplexer, and decoding the received control signal, thereby controlling an ON/OFF mode of the control signal decoder,
a first inverse quantizer for receiving the variable-length-decoded quantized transform coefficient block, and inverse-quantizing the received transform coefficient block,
inverse orthogonal transform means for selectively receiving the inverse-quantized transform coefficient block from the first inverse quantizer while receiving the shape information from the overhead information decoder and the transform direction control signal from the control signal decoder, and inverse-orthogonal-transforming the inverse-quantized transform coefficient block in a direction inverse to that of the orthogonal transform means,
an adder for receiving the inverse-orthogonal-transformed signal block from the inverse orthogonal transform means while receiving an image signal block previously recovered, and adding the received signal blocks, thereby outputting a recovered image signal block, and
a memory for storing the recovered image signal block outputted from the adder, and outputting the stored imaged signal block to the adder for an prediction.
In accordance with another aspect, the present invention provides an image compression-encoding method for performing an orthogonal transform for blocks of an input image signal in accordance with an adaptive transform method, thereby compressing the input image signal blocks, comprising: a transform direction control signal generating step for selectively receiving transform coefficients, obtained after an orthogonal transform for blocks of an input image signal, and an image signal block to be currently encoded, determining correlations of the input values in horizontal and vertical directions, and generating, based on the determined correlations, a transform direction control signal adapted to control an orthogonal transform for the current image signal block in such a fashion that the current image signal block is orthogonal-transformed in a direction involving a higher correlation, and then in a inverse direction; and an orthogonal-transforming step for receiving the transform direction control signal, and performing an orthogonal transform, in a sequential fashion, based on the received transform direction control signal.
In accordance with another aspect, the present invention provides an image compression-encoding method for decoding a compression-encoded image signal, transmitted, using an adaptive transform method comprising: a inverse transform direction control signal generating step for selectively receiving a signal block, currently variable-length-encoded, and signal blocks already encoded, determining correlations of the received blocks in horizontal and vertical directions, and generating, based on the determined correlations, a inverse transform direction control signal adapted to control a inverse orthogonal transform for the current signal block in such a fashion that the current signal block is inverse-orthogonal-transformed in a direction involving a lower correlation, and then in a inverse direction; and a inverse-orthogonal-transforming step for receiving the inverse transform direction control signal, and performing a inverse orthogonal transform, in a sequential fashion, based on the received inverse transform direction control signal.
In accordance with another aspect, the present invention provides an image compression-encoding and decoding method for encoding and decoding input image signal blocks, comprising: a transform direction control signal generating step for generating a transform direction control signal adapted to determine respective direction orders of an orthogonal transform and a inverse orthogonal transform, based on an image signal block, to be encoded, and shape information; an encoding step for encoding the transform direction control signal generated at the transform direction control signal generating step while orthogonal-transforming the image signal block, to be encoded, based on the transform direction control signal, quantizing the orthogonal-transformed signal, and variable-length-encoding the quantized signal; a transmitting step for multiplexing the encoded transform direction control signal and the variable-length-encoded signal block, and transmitting the resultant signal; a signal separating step for receiving the encoded signal transmitted at the transmitting step, and demultiplexing the received signal into the encoded transform direction control signal and the variable-length-encoded image signal block; decoding the encoded transform direction control signal separated at the signal separating step, variable-length-decoding the variable-length-encoded image signal block, and inverse-quantizing the decoded image signal block; and a signal recovering step for inverse-orthogonal-transforming the inverse-quantized signal block in a state in which the inverse orthogonal transform is determined in direction order, based on the decoded transform direction control signal, and recovering the inverse-orthogonal-transformed signal block into the initially inputted image signal block, based on an image signal block previously recovered.